Conventional digital oscilloscopes acquire, record, and display amplitude vs. time waveforms. Samples of an input signal are taken and quantized, and the resultant digital representations are stored in a memory under the control of a sampling clock. The data thus acquired is subsequently clocked out of memory and converted to an analog replica under the control of a display clock. The sampling clock may be operated at one of several selectable rates, depending upon the frequency content of the input signal.
A problem arises in repetitive-waveform acquisition /display systems in that the triggering point, which is the same on each successive waveform, and the sample clock, which operates at a predetermined fixed rate, are not correlated. Unlike a conventional analog oscilloscope wherein an event-related trigger initiates a time-base sweep, the display in a digital oscilloscope begins on a clock signal. Since the trigger point on a waveform may fall anywhere between two successive sample clock pulses, there exists an uncertainty of .+-. one-half sample interval with respect to the trigger point. In terms of a repetitive-frame display, the sample uncertainty is manifested as horizontal jitter of the waveform of .+-. one-half sample clock period. The observable jitter may vary from barely discernible to intolerable, depending upon the various clock and signal rates involved. As a rule, however, the observable jitter increases as the sample density, i.e., number of samples per cycle, is decreased. For example, assuming a display time-base length of 10 scale divisions, repetitive waveforms sampled at a 5-megahertz rate (200 nano-second sample clock period) and subsequently displayed at the equivalent of 20-nanosecond per division rate will have horizontal jitter of .+-. one-half division, or one full division overall, thereby rendering such a display unitelligible as far as any time measurement is concerned.